Pharm Basics

Drug Interactions

Pharmacology Basics

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A drug interaction occurs when one substance changes the effect of another. They can increase toxicity or decrease therapeutic effect.

Feature
Pharmacokinetic
Pharmacodynamic
What it affects
ADME (absorption, distribution, metabolism, excretion)
Action at the receptor or system level
Mechanism
Changes the AMOUNT of drug in the body
Changes the EFFECT of the drug
Example
Antacids ↓ absorption of tetracyclines
Opioids + benzodiazepines → additive CNS depression

Dangerous Combinations

  • Warfarin + NSAIDs / aspirin↑ bleeding risk
  • SSRIs + MAOIs / tramadol / linezolidSerotonin syndrome
  • ACE inhibitors + K-sparing diureticsHyperkalemia → arrhythmias
  • Macrolides + fluoroquinolones + antipsychoticsQT prolongation → torsades
  • Opioids + benzodiazepines + alcoholRespiratory depression, death
  • Rifampin + oral contraceptivesContraceptive failure (CYP450 induction)

Food–Drug Interactions

Grapefruit juice + Statins, CCBs, cyclosporine

↑ drug levels (CYP3A4 inhibition) → toxicity

Leafy greens (vitamin K) + Warfarin

↓ anticoagulant effect — keep intake consistent

Dairy / antacids (Ca, Mg, Al) + Tetracyclines, fluoroquinolones

Chelation → ↓ absorption

Tyramine-rich foods (aged cheese, wine) + MAOIs

Hypertensive crisis

High-fiber meals + Digoxin, levothyroxine

↓ absorption

Key Pearls

  • Always reconcile ALL meds — including OTCs, herbs, and supplements
  • Stagger oral doses when absorption interactions exist (separate by 2 hours)
  • St. John's Wort is a major CYP450 inducer → reduces many drug levels
  • Recognize serotonin syndrome: agitation, hyperreflexia, hyperthermia, clonus

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Disclaimer: PharmPal Nurse is for education only and is not medical advice. Do not use it to diagnose, treat, prescribe, or make patient-care decisions; always verify with current drug references, your instructor, facility policy, and a licensed provider. In emergencies, call local emergency services.